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We asked for your questions last week after features on the vision-correcting surgical procedures Lasik and radial keratotomy.

Here, Dr. Rex Hamilton, assistant professor of ophthalmology at the University of California, Los Angeles, and Dr. James Salz, clinical professor of ophthalmology at the University of Southern California, discuss Lasik's success rate, side effects, and the need to tailor expectations to the condition of your eye:

NPR: A number of people wrote in saying they had Lasik within the last three to seven years and were troubled by how much their vision has regressed. Some say they're back in glasses or contacts, while others say they're back to their original prescriptions. How typical is such vision regression?

If people say their vision has gotten worse, most of the time they're talking about their reading vision, primarily because they're getting older. Deterioration in reading vision happens to everyone around age 40 to 45. If what they really mean is that they have become more nearsighted again, that's pretty rare; that's maybe 5 percent of Lasik patients. That 5 percent refers to a significant change, about a -1 diopter or more, over a 10-year period. Many of these patients can successfully undergo an enhancement procedure to get rid of that residual nearsightedness.

I should point out that with the technology used in Lasik more than five years ago, there was a higher incidence of regression because of the way the cornea was reshaped.

It's highly unusual for someone's vision to revert to the original prescription. I've never seen that, and if that did occur, it could indicate that other things are going on with the eye, such as a cataract. Distance vision should not drift appreciably once you've gotten to your original target vision with Lasik.

The most common risk factor for regression is dry eyes. If you have dry eyes and it goes untreated, you can experience some regression, or loss of effect from the surgery.

I've always been told that because I am so nearsighted (-10.5 each eye) that Lasik surgery would not be able to correct my vision fully. Is this still true with this new form? — Liz Lee, Houston, Texas

That may have been a problem with older technology, but I think the bigger issue at -10 diopters is the safety of the procedure at that level of nearsightedness. In my opinion, it's rare that I feel that someone's cornea is appropriate for Lasik at that level, because it usually requires removal of too much corneal tissue. There's certainly a higher chance that someone at that level will need an enhancement, and the problem is there may not be enough tissue left to do the enhancement.

There's a newer option for people at that level of nearsightedness and it involves putting a lens inside of the eye. It's called a phakic intraocular lens implant. That concept has been around for about 10 or 15 years in Europe. The FDA approved two lenses this past year for use in the United States (Verisyse and Visian ICL). It's a better option for high nearsighted corrections, because we don't have to alter the thickness or shape of the cornea. Consequently, we don't have to worry about the cornea becoming too weak down the road. Also, at that level of nearsightedness, the quality of vision will be better if you put a lens inside the eye rather than drastically altering the shape of the cornea.

Dr. Jim Salz agrees that the phakic intraocular lens implant is a better option when a high level of correction is needed. He adds:

There is a low but serious risk with this surgery, because you are working inside the eye. If you get an infection when putting one of these implants in, you could lose your sight. This hasn't happened so far in the clinical trials, but we don't really know the risk of infection. It has to be less than one per 3,000 to 4,000 cases, because that's the risk of infection with cataract surgery, which is a similar procedure. And the risk should actually be less, because the lens implant is a less invasive procedure and done on younger, healthier patients. You can get an excellent opinion in Houston at Baylor.

I had Lasik seven years ago. Can an enhancement be done to improve my sight and get rid of the halo and bad nighttime vision using this new procedure? — Randi McFadden, Richmond, Va.

Yes, and I've done that, but candidates need to have a full evaluation. They need to have enough cornea tissue left for the procedure to be safe.

Presbyopia is the natural loss of focusing power of the lens of the eye. When people need reading glasses at 45 or older, that's presbyopia at work. To correct that, you have to do something to the lens of the eye, and Lasik works on the cornea, not the lens. But there are things we do with Lasik to treat the effects of presbyopia, such as monovision or blended vision, where we treat one eye for distance and one eye for near vision. It can hold off the need for reading glasses, but the duration of effect depends on each patient's occupational and recreational vision needs. A fair number of people with this surgically created monovision will find they still need glasses for at least one thing in their life, and most commonly, that's night driving.

I've been told the thickness of my cornea is at a borderline level and that I am not a good candidate for Lasik. Is cornea thickness still a factor? — Christine Kosmowski, Battle Creek, Mich.

Corneal thickness is definitely very important, but it's not the be-all and end-all. The shape is very important as well. At the UCLA Laser Refractive Center, we are using a new device during our screening process that tests the strength of the cornea by pushing on the cornea with a very short puff of air. All of these components of the screening examination — thickness, shape and strength — are extremely important. You could have normal thickness, but the shape is irregular and so you're not a great candidate. But if the cornea is thin, regardless of the shape, I'm not going to do Lasik. Those with thin corneas typically are a candidate for PRK (photorefractive keratotomy), which is the same laser shaping, but without the flap. PRK has excellent long-term results but the post-operative recovery period is longer than with LASIK.

My eye doctor is concerned that Lasik has not been around long enough for us to know the magnitude of any side effects. How likely are side effects? — Vickie Choitz, Somerville, Mass.

Dry eyes are the most common side effect, typically lasting three to six months after the surgery. We know that dry eyes are more common in women than men, and it's more common in postmenopausal women than in premenopausal women. But there are very effective treatments, including a medicated eye drop called Restasis and oral supplements called BioTears.

The other side effect you will hear about is quality of vision at night, such as halos and glares off of lights. This used to be a problem, especially in higher corrections. In my experience at the UCLA Laser Refractive Center, it's much less of an issue with custom wavefront Lasik now.

The most recent numbers I have seen are about 15 percent of Lasik cases use laser flap creation (called intralase) instead of a blade, and that number is going up. The intralase system is expensive, so laser centers have to perform a fairly high volume of surgery to make that work financially, but there's more and more evidence that it is very useful and provides benefits in terms of safety and predictability of results. Wavefront-guided Lasik is on the order of 40 to 50 percent and also going up.

Yes, with limitations. Most of the FDA lasers are approved for up to about 4 diopters of astigmatism correction, which covers 98 percent of the population. Some astigmatism irregularities indicate a problem with the cornea, which indicates you're not a great candidate for refractive surgery.

I have dry eyes and have been told that Lasik surgery would make this condition worse. Is it possible that new Lasik technology might someday make this surgery an option for me? — Sunny Monroe, Bothell, Wash.

The bottom line is that dry eye needs to be treated prior to surgery if it's a pre-existing condition, and we have the tools now that allow us to treat dry eye in most patients.

As for new technologies, it's possible that Lasik with the laser-created flap may cause less of a problem with dry eye, because you can create a shallower flap. The deeper the flap, the more corneal nerves are affected by the surgery. These nerves sense dryness and tell the tear glands to make more tears. So if you cut through those, they don't work for a while and need time to regenerate. So the shallower the flap, the less effect.

How often does laser surgery need to be performed to be maintained? And what is the success rate? — Marisa Bodell, Santa Fe, N.M.

Ten percent of patients have to have enhancements or touchups one time. This is typically determined within the first year after surgery. The higher the correction, the higher the chance of the need for an enhancement.

Dr. Salz adds:

For up to about – 8 diopters nearsightedness and up to about + 5 diopters farsightedness, the success is very high, with more than 90 percent of patients achieving better than 20/40 vision. If we did the surgery on a stable eye, the surgery will last. We don't give you a warranty that your vision isn't going to change, but the majority of patients I've operated on have had one surgery and they never come back.

With wavefront-based Lasik, most studies show that more than 90 percent of patients obtain 20/20 vision or better.

What steps can I take to ensure the safest possible experience for laser surgery? — Peter Lowe, Stamford, Conn.

Go to a reputable laser center focused on the patient and not on volume. I think that major institutions, major universities are always focused on the patient; I don't know of any university that is performing high volume surgery, e.g. 30 to 40 patients a day. Lasers need to be recalibrated during the surgery day to ensure accurate treatments. We recalibrate our laser between each eye of each patient to ensure the most accurate results. In addition, high volume means a higher chance of something being overlooked. Also, you want to be able to meet your surgeon before the day of surgery, particularly before you're on the table.

There's not a lot you need to do preoperatively outside of not wearing eye makeup, perfumes or colognes. If you're wearing soft contact lenses, stop wearing them anywhere from three to five days before your screening. Rigid lenses need to be out for weeks, depending on how long you've worn them.

Dr. Jim Salz, clinical professor of ophthalmology at USC, answers the following questions:

PRK was the first laser procedure approved, and still gives excellent results.

With PRK, we don't make a flap; we take the surface skin (epithelial layer) off the cornea. You can take this layer of cells off by rubbing it or loosening it with alcohol, and then you apply the laser to the next layer. The laser part of the surgery is exactly like Lasik. Then you put a bandage contact lens on to protect the eye while this epithelial layer grows back, and the eye has the same chance of good vision as with Lasik.

It's a little bit safer than Lasik because there's no flap, but the reason we don't do that operation more often is it takes longer for the eye to heal, and there's more discomfort in those initial three days than with Lasik. The Navy uses PRK on its pilots, but not Lasik. The thinking is if you have to be ejected from a plane, the forces that might hit the eye could displace the flap. But other military people can have either LASIK or PRK.

I had radial keratotomy surgery in 1995, and my vision has regressed, with significant fluctuations in my vision from morning to night. What are my options? — Doug Wagner, Sherman, Ill.

You can have surface PRK with good results but these vision fluctuations will most likely continue. Called diurnal variation, these vision changes are very common with RK and unheard of with Lasik.

RK, performed in the '80s and early '90s, used a blade to make a series of incisions radiating from the center of the cornea. The procedure turned out to be unstable and most RK patients end up with blurred vision for both near- and far sight. Some of the RK eyes eventually stabilize but some keep changing, so the PRK can be repeated again if necessary. I no longer like to do LASIK in RK eyes because it is riskier, because you have these incisions from the original RK surgery to deal with. The Lasik flap would have to cut through these RK incisions and it's a complication you'd rather not have to deal with.

If you have had radial keratotomy and cataract surgery, can you still have the Lasik eye surgery? — Joyce Beshears, Fort Smith, Ark.

Yes. The best option would be to have PRK to get the vision you want.

People who've had RK and then have cataract surgery to replace their cloudy lens have a greater chance of getting an inaccurate lens implant, i.e. the implant doesn't correct the vision to the degree desired. But we don't like going back into the eye to replace the implant. It's less risky to fix the difference with PRK or Lasik.

Is all-laser Lasik surgery safe for glaucoma patients and people at risk for glaucoma? Is glaucoma more difficult to manage after Lasik? — Mary King, Brant Lake, N.Y.

It can be safe to do unless there is already a lot of optic-nerve damage. The glaucoma can be managed but the pressure reading by the doctor has to be adjusted after LASIK or PRK as the pressure tends to read lower than it actually is.

It is probably one of the safest surgeries ever developed, but it is still surgery and carries a small risk of serious complications. The most serious complication would be an infection under the Lasik flap. If it's a minor infection and not in the center of the cornea, we treat it with antibiotics and there's usually no permanent damage. In rare cases, when the infection is in the center of cornea, and it's a particularly virulent bug, it can be more difficult to control and the infection can leave scarring and your vision will be blurred. At that point, you'll have to have a cornea transplant to fix the blurring, but that happens in about one in 100,000 cases. It's pretty rare.

I have heard that Canada is far ahead of us in performing Lasik eye surgery, and we should go to Canada for it because the cost is less and the eye surgeons have more experience plus the latest procedures. Can you comment? — Helaine Rocket

Our FDA is very tough when it comes to getting devices approved, and for a while, Canada was able to do some laser procedures before they were available in the U.S., but this is rare now. I doubt if it is much cheaper in Canada because of the declining value of the U.S. dollar.

Is there a best age for which to have surgery? — Debra Dolly, Clinton, Mo.

Age isn't as important as stability of vision. You need a couple of years with essentially the same prescription and that usually happens for most people in the mid-20s. I have operated on 70-year-olds and even — rarely — on 18-year-olds. For someone that young, they have to have records showing exactly the same prescription for four years. For the most part, we don't do Lasik on children, but there have been some unusual circumstances.